The present invention relates to an improvement in forming ceramic cellular articles, such as honeycombs, from an extrudable or moldable powder batch composition comprising an inorganic powder component consisting of a mixture of sinterable inorganic particulate materials, e.g., ceramic powder materials, and a binder system including oil-based compounds which are flammable during subsequent de-binding. In particular the improvement comprises extracting the oil-based components from green structures with a solvent prior to firing.
Popular ceramic multicellular ceramic bodies which are generally formed by extrusion, such as those finding applications in catalytic converters, diesel particulate filters, electrically heated catalysts, and chemical processing, require binders and extrusion aids for proper processing. Typically, water-soluble cellulose ether binders are used; however, such binders alone are not suitable for the manufacture of extremely thin-walled, high cell density cellular structures, and webs of 4 mils or less. U.S. Pat. Nos. 6,080,345 and 6,368,992 disclose the use of oil-based compounds, such as polyalpha olefin, in combination with cellulose ether binders for improved processing of such structures. The use of oil-based components improves extrudability and wet strength of the green structures. For ease of description oil-based compounds shall be referred to as oil or oils hereinafter.
While advantageous for extrusion, oils can lead to significant problems during firing. Specifically, such components cause a strong exotherm, which unless controlled effectively, can lead to uncontrolled combustion or detonation in the kiln. Significant amounts of small volatile combustibles are released during firing resulting in pressure build-up inside the kiln. In an inert atmosphere, and at temperatures above their boiling points, the oils evaporate quickly leading to sudden local heat losses, thermal stresses, and ultimately cracks in the ware.
Specially designed kilns, apparati for volatile removal, and elaborate firing cycles have been employed to control the burnout of oils, and reduce combustible concentrations in the kiln atmosphere. Closed kilns must be effectively managed so as not to cause safety issues. The reduced kiln throughput and significant costs associated with safe operation are operating issues of significance.
Removal of oils from structures, including ceramic bodies, is known in the prior art. Co-pending U.S. Pat. App. having Ser. No. 60/422,191, entitled “Process for removing oil-based components and forming ceramic bodies”, by Chan et al., discloses a method of forming cellular ceramic bodies which includes removing oils from green bodies, after drying and prior to firing, by flowing a heated gas through the green body to evaporate the oils.
U.S. Pat. No. 3,904,551 issued to Lundsager et al. is directed to a method of preparing an auto exhaust catalyst converter from a mixture of ceramic powders, a polyolefin binder component and a plasticizer component. Therefore, this reference specifically teaches the use of non-aqueous thermoplastic binder systems. Prior to firing, the plasticizer component, e.g., mineral oil, is removed by a solvent, e.g., hexane, to form porosity and a microporous structure. Structures of cordierite are disclosed. However, cordierite bodies made from a mixture of inorganic powders and a water-soluble binder system, which may additionally contain a surfactant or other processing aids, are not recognized. In addition the advantages of using oils to improve extrudability and wet strength are not taught. Further, removal of oil-based components by selective dissolution thereof without dissolving the organic water-soluble binders is not taught. Moreover, the effect of removing oils on green strength, and key fired chemical, physical, as well as mechanical properties is also not recognized.
Similarly, U.S. Pat. Nos. 3,755,204, 3,854,186, 3,926,851, and 3,911,070 disclose the removal of oils from ceramic structures, including cordierite, by using a solvent. Again, the ceramic cordierite bodies do not include a combination of a water-soluble binder, and oil-based components, and there is no recognition of the benefits of improved extrudability and wet strength by using such water-soluble binder and oil combinations. Further, there is no discussion on selective removal of the oil-based component, while keeping the water-soluble binder intact, and maintaining high green strength as well as key fired chemical, physical and mechanical properties.